IPv4 network addresses
To communicate with a device on the network, it must be equipped with an IP address. Currently, the addressing technology we use is Internet Protocol version 4 (IPv4) addressing. The label v4 represents the fourth version of IP addressing, as specified in IETF publication RCF 791. However, you will often encounter it as simply an IP address. It is a logical element that consists of 32 bits and identifies a network interface of a given device. It is organized in 4 octets with 8 bits in each of them, divided by a decimal point for simplicity of interpretation (for example, 192.168.1.1). If we take into account the fact that 1 byte = 8 bits, then a 32-bit IP address is 4 bytes (that comes from 32/8 = 4). Additionally, IETF's RFC 791 document manages IPv4 addresses into prefixes of 8-bit, 16-bit, or 24-bit. This enabled organizing IPv4 addresses into classes of A, B, C, D, and E, which is known as classful addressing. Classful addressing defines the bits used for the network portion and bits used for the host portion for a given class. It is obvious then that in the case of IP addresses, everything revolves around the binary numbering system. Thus, the total number of IPv4 addresses is 232 = 4,294,967,296.
If you compare that number to the number of the world population, it is obvious that nearly three billion IP addresses are missing if an IPv4 address is given to every person.